Electrical energy storage systems or secondary batteries of the so-called high energy density (H.E.D.) type have assumed increasing importance in recent years because of their capability of supplying upwards of 50 watt hours of electrical energy per pound of weight. The high energy capacity and compactness of such H.E.D. batteries renders them particularly satisfactory for use as principal or auxiliary sources of electrical energy in both mobile and stationary power plant systems. A metal/halogen/hydrate secondary storage battery of the foregoing type is described in detail in U.S. Pat. No. 3,713,888, the teachings of which are hereby incorporated by reference.
During the charging of high energy density secondary batteries using chlorine and a highly electropositive metal as electrodes and reactants, an aqueous metal chloride salt electrolyte is circulated through the battery cells, plating out the metal on an electrode base and generating chlorine at the other electrode. In cells where the electrodes are of zinc and chlorine on a carbon base, an aqueous zinc chloride electrolyte is used and continuously circulates through the cells during the charging and discharging operations. Zinc is deposited on the active surface of the zinc electrode during charging, and is consumed during discharging.
The quality of the zinc deposited on the zinc electrode during the charging operation is prone to a number of quality defects which, in turn, detract from the performance of the battery. Two of the principal quality defects are dendrites, i.e., protuberances out of the surface of the zinc, and a low density semi-amorphous zinc which is known as "black zinc". Because the spacing between electrodes is small, as it must be, the growth of dendrites and black zinc causes shorting within the battery, can cause increased hydrogen evolution and result in lowered efficiency. It is apparent that avoidance of the formation of dendrites and of black zinc is desirable. Unfortunately, dendrite formation and black zinc formation generally act as if they were in equilibrium with each other. The higher the charge current density, the greater the tendency to form dendrites and, conversely, the lower the charge current density, the greater the tendency to form black zinc. There has not been a current density which is low enough to avoid dendrites, yet high enough to avoid black zinc formation.
Another problem with the electrodes heretofore used in the zinc-chlorine H.E.D. battery is that conventional flat plate electrodes and electrode arrangements are not particularly well suited for use in a battery where the electrolyte circulates during charging and discharging. Electrode material such as graphite is easily deformed and broken into thin sections, particulary if substantial pressure and pressure differentials are applied to the material. In the flowing electrolyte zinc-chlorine battery, the electrode material of choice is graphite and both substantial pressures and pressure differences are encountered. Furthermore, the fabrication of electrodes with large flat areas, and with a plurality of parallel channels in one surface of the electrode is complex and expensive.
It has been found that the foregoing problems and deficiencies can be substantially avoided if zinc electrodes used in the zinc-chlorine battery are constructed in the new and improved manner of the present invention. In a preferred embodiment, each of the electrodes are essentially identical to one another and are each substantially rod-like shaped and are constructed of an electrolyte porous material and have a central axially directed electrolyte flow channel extending completely therethrough, or extending at least from one point on the electrode end surface into the interior of the electrode.
The simple rod shape of each of the electrodes permits simple and inexpensive fabrication of the electrodes, and they are interchangeable with one another and are symmetrical to permit easy assembly of the battery. Any number of rod pairs may then be mounted between spaced, parallel conductive plates to form a battery which can withstand high mechanical stress without damage to the sturdy rod-shaped electrodes.
U.S. Pat. No. 3,813,301 discloses a bipolar electrode having two sides joined in such a way as to form a channel for flow of electrolyte therebetween. The front side of one bipolar electrode and the back side of another bipolar electrode form an electrolytic cell couple and the patent recommends that additives be employed to avoid formation of dendrites.
It is the object of this invention to provide a new and improved electrode and electrode system for use in a zinc-chlorine flowing electrolyte high energy density battery which is better adapted to withstand the stresses and strains in use and can be controlled so as to avoid significant formation of dendrites and black zinc and which simplifies the construction and assembly of the battery. Other objects of the invention will become apparent to those skilled in this art from the following detailed description in which: